This invention relates to franking machines for applying a franking impression to mail items.
Commonly used franking machines include a mechanical printing device consisting of settable print wheels housed in a print drum. The print drum carries on its surface a raised pattern of fixed information to be printed and print characters on the peripheries of the print wheels are utilised to print variable information such as the value of franking and the date. When it is desired to effect franking of a mail item, the print wheels are rotated to bring print characters thereon, corresponding to the required value of franking, into an operative printing position in which the required print characters are positioned to project through an aperture in the wall of the print drum. The drum is then caused to rotate through one revolution during which the raised pattern on the drum and the operative print characters are brought firstly into contact with an inking device and then into contact with the mail item which is pressed against the drum by a pressure roller so that the main item is fed with the rotation of the drum at the same linear speed as that of the peripheral surface of the drum.
The print wheels may be set to the required value of printing by means of manually operable levers or sliders devices which are mechanically coupled to the print wheels. Alternatively the print wheels may be mechanically coupled of electric motors which are driven under the control of electrical signals from a keyboard. In both of these constructions the print wheel setting requires the provision of relatively complex mechanical linkages to ensure precise setting of the print wheels. In addition, for accounting purposes the value to which the print wheels are set needs to be input to an accounting device in the franking machine. In present franking machines the accounting device usually consists of electronic circuits including a microprocessor and data storage registers. Accordingly is is necessary to provide electro-mechanical transducers to convert the mechanical setting of the print wheels into electrical signals which can be utilised by the electronic accounting circuits.
It has been poroposed to use thermal transfer printing devices in a franking machine. In thermal transfer printing a ribbon having an ink coating adhered a plurality of selectively heatable print elements and a mail item and upon heating of selected print elements the ink on the ribbon adjacent those heated elements is transferred from the ribbon to the surface of the mail item to form a desired printed pattern. The printing elements of such a thermal transfer printer are heated by the passage of electrical current therethrough and the routing of electrical current to the selected elements is carried out by electronic switching circuits. As a result the selection of printing elements to print a required value of franking does not required setting of mechanical elements or the conversion from mechanical setting to corresponding electrical signals.
Both mechanical impression printing and thermal transfer printing as described hereinbefore require the surface of the mail item to be urged into intimate contact respectively with the printing elements and the transfer ribbon. Mail items may vary substantially in thickness and hence difficulties arise in ensuring the required intimate contact. Accordingly it would be desirable to utilise a method of printing which does not require such contact with the surface of the mail item.